1. Field of the Invention
The present invention relates to a fixing device of an image forming apparatus that uses an electrophotographic method or an electrostatic recording method. Examples of the image forming apparatus include printers, such as laser beam printers and LED printers, and digital copiers.
2. Description of the Related Art
A film is used in some fixing devices of electrophotographic image forming apparatuses. Typically, a fixing device of this type includes a film, a heater that is in contact with the film and that heats the film, and a backup member that is in contact with the film and that forms a nip for transporting a recording medium. The fixing device, which uses a film having a small heat capacity, requires only a short warm-up time and has a merit in that it can contribute to reduction of a first print out time (FPOT).
On the other hand, it is known that the fixing device using a film tends to cause a so-called “temperature rise of sheet-non-passing area”, that is, a phenomenon in which the temperature of the sheet-non-passing area is increased considerably when a recording medium having a width smaller than the largest size (hereinafter referred to as a small-sized recording medium) is passed through the fixing device. In order to suppress the temperature rise of sheet-non-passing area, Japanese Patent Laid-Open No. 11-84919 describes a structure in which a metal plate is in contact with the heater.
With the structure described in Japanese Patent Laid-Open No. 11-84919, the temperature rise of sheet-non-passing area can be suppressed due to the function of the metal plate, which is in contact with the heater over the entire length of the heater. However, when warming up the fixing device, a decrease of the temperature of an end portion of the heater in the longitudinal direction (hereinafter referred to as “end portion temperature decrease”) tends to occur. Due to the end portion temperature decrease, this structure has a problem in that, when fixing is performed on a recording medium having a large size, a fixing failure may occur at the end portion of the recording medium.